The invention relates to a device in which bands guided in a suspended manner, preferably wide metal bands, are held in suspension by being blown with treatment gas from above and below and are thereby simultaneously heat treated. Heat treatment may involve heating and maintaining the annealing temperature as well as subsequent cooling for metallurgical purposes or also for the purpose of surface treatment such as, e.g., drying a band coating.
Such devices are configured as a rule of a plurality of sections or zones joined in sequence, and including, as is the case in, for example, German published patent specifications DE 2446983.8 or DE 4010280 A1, a flow guide for each section, having at least one ventilator and nozzle fins arranged above and below the band, oriented transversely to the run of the band, with which the band is blown with the gas circulated by the ventilator for the purpose of convective heat exchange, and, at the same time, stabilized more or less effectively by the active flowing forces. In particular, devices for operation at elevated gas temperatures are usually equipped with radial ventilators for reasons relating to adequate structural strength.
Although the device in accordance with specification DE-OS 2446983.8 has a particularly compact design as only one ventilator is arranged at the side per section, simply regulation of the supporting force by altering the ventilator speed, e.g. to adapt to the weight of the band to be guided in a suspended manner, is not possible since the single ventilator supplies both upper and lower nozzle sections simultaneously and this would influence upper and lower nozzle systems alike. In addition, the device according to specification DE-OS 2446983.8 has the disadvantage that the return flow of the treatment gas, blown onto the band, occurs only on one side to the ventilator. This causes a cross-flow heat exchange between the return flow from the band and the supply flow in nozzle fins, with the result that, e.g., in a heating zone, in which the gas blowing off the band is colder than that blown onto the band, the temperature of the blown gas decreases along the bank of nozzles from the side facing away from the ventilator to the ventilator side.
Although the device according to German published patent specification DE 40120280 A1 avoids this disadvantage by alternating the supply flow to the bank of nozzles within a section, it is, however, highly complicated and expensive to manufacture, due to the nozzle fins, which also need to be provided with a complicated arrangement of flow duct means in order to achieve a flow impinging the band precisely perpendicular over the entire working width. Furthermore, using nozzle fins in a device in accordance with DE 40120280 A1 also has the disadvantage that, in the case of heavy bands, the total return flow from the band can only occur between the nozzle fins because the side regions of the device are blocked by the supply ducts which feed the nozzle fins. As a result, the overpressure needed for supporting the band from below is built up again, stripwise, i.e. between the nozzle fins, by the convective acceleration of the air between the nozzle fins. Thus, boosting the nozzle exit velocity results in an increase in the supporting force only in part, since, to extreme disadvantage, the convective acceleration between the nozzle fins likewise increases with the increase in the jetting velocity and the resulting increase in the overpressure on the product web in the region of the bank of nozzles. This results in a drop in the static pressure in this region, as a result of which most of the boost in the supporting force, gained by increasing the velocity, is again forfeited. The consequences are relatively high ventilator power requirements and the supporting force being restricted to unit weights, which, in the case of an annealing plant for metal bands, are insufficient to guide heavy non-ferrous metal bands, steel bands or light metal alloy bands of greater thickness.
An object of the invention is thus to provide a device of the aforementioned kind, which obviates the disadvantages described. More particularly, the intention is to provide a device which is relatively uncomplicated and compact and which avoids the disadvantages of returning the gas stream, blown by the bank of nozzles onto the band which is to be guided in a suspended manner, only through intermediate spaces between the nozzle fins.
This is achieved by the features as set forth in claim 1, The sub-claims describe advantageous aspects. Preferred embodiments feature, for example the following features:
The return flow from the band in the suction zones of the radial ventilators is made primarily to both sides of the device.
Arranged above and below the band are nozzle sections, each respectively extending over the entire working width and the entire length of the device, each respective nozzle section receiving blown gas from a collecting receptacle located above or below the nozzle section.
Incorporated in each side wall, above and below the plane of the band, is a radial ventilator, preferably a spool rotor ventilator, accommodated in a 360xc2x0 spiral casing. Connecting to the 360xc2x0 spiral casing is an air delivery channel, which is at least as long as a hydraulic diameter of the duct running parallel to the respective side wall. Each of the intake openings of the ventilators point toward the vertical longitudinal center plane of the device.
Connecting each air delivery channel is a first elbow running parallel to the plane of the band. This first elbow ports into a second elbow, which is parallel to the vertical section through the device, this second elbow translating into a vertical duct, from which the collecting receptacle or supply chamber is fed for each nozzle section.
Due to the air delivery channels of two opposing radial ventilators being arranged opposing each other in one half of the device, above or below the plane of the band, the supply chamber of the nozzle section is fed via two inlet openings in each face wall of the device.
The free space remaining between the ventilator spiral casing and the face wall of the device may be used as an access opening in the corresponding upper or lower part of the device or for installing heating means.
By arranging the ventilators with a spiral casing with the intake opening adjoining the top or bottom of the device, a free space remains between the collecting receptacle or supply chamber for the nozzle section and air delivery channel of the corresponding side wall, in which openings for installing burners, radiant heating tubes and similar also exist.
Coolers for cooling the gas stream, circulated by the ventilators, may be incorporated parallel to the collecting receptacles for the nozzle section on both sides of the nozzle boxes relative to the side wall, i.e. two in the upper and two in the lower half of the device.
The coolers may be accommodated in collecting receptacles, integrated in the outer wall, so that the same device may be used to both heat and cool the band, the coolers being taken out of circuit when the device is being operated as a heating zone, i.e. for heating the band. The face wall of the cooler assembly, facing the interior, is then provided with a suitable thermal insulation.
The nozzle section may consist of individual part nozzles interconnected by semi-open ducts to facilitate the predominantly lateral flow-off. However, a nozzle box with a nozzle plane may also be used, incorporating flow-off orifices penetrating the box in the direction perpendicular to the plane of the band. The flow-off orifices may be dimensioned such that, depending on the unit weight to be supported, a more or less large gas volume exits through the flow-off orifices and a more or less large proportion of the gas stream blown onto the band flows off laterally, the ratio of lateral flow-off to the flow-off through the openings penetrating the nozzle box being involved in dictating the level at which the band is to be suspended.
In the following, the device in accordance with the invention is described by way of a heat treatment system for guiding wide metal bands in a suspended manner as an example. Such a wide band is 1000 mm to 2500 mm wide. FIGS. 1 to 6 aid explanation of the invention.